It is well known that when a silver halide color light-sensitive material is exposed, an oxidized aromatic primary amine developer reacts with a dye-forming coupler, producing a dye image. In this method, color reproduction is usually carried out by the subtractive color process: i.e., dyes of cyan, magenta and yellow in complementary relation with red, green and blue, respectively, are formed. The reaction between the oxidized developing agent and the coupler occurs at an active site of the coupler.
A coupler having a hydrogen atom at the active site is a four equivalent coupler; i.e., it needs as an oxidizing agent 4 moles of silver halide with a developing nucleus in the formation of 1 mole of dye. On the other hand, a coupler having a releasing group as an anion at the active site is a two equivalent coupler; i.e., it requires only 2 moles of silver halide with a developing nucleus for the formation of 1 mole of dye. Therefore, when a two equivalent coupler is used, the amount of silver halide in a light-sensitive material and film thickness can be decreased compared with when the four equivalent coupler is used. Hence the use of the two equivalent coupler makes it possible to shorten the processing period of the light-sensitive material and further increases the sharpness of a color image formed. Furthermore, the coupling activity of the two equivalent coupler to the developing agent can be changed widely by suitably selecting the type of releasing group.
A two equivalent coupler releasing a compound capable of inhibiting development is called a "development inhibitor releasing coupler (DIR)". Since this coupler inhibits development in proportion to the amount of developed silver, it produces various effects, such as formation of fine grain images, control of gradation, and improvement of color reproductivity. Furthermore, the coupler can be used in the diffusion transfer process in view of its action on an adjacent layer.
By providing a releasing group containing a diffusing dye portion, the resulting coupler can be utilized in the diffusion transfer process in which the dye released is allowed to diffuse in an image-receiving layer and transferred thereto to form a dye image. Couplers of this type are called "diffusing dye-releasing couplers".
A certain kind of colored two equivalent coupler has the mask effect for color correction of a dye image. Couplers of this type are called "colored couplers".
As described above, the two equivalent coupler can be provided with various functions by suitably selecting the type of releasing group.
There are two general trends in the recent development of silver halide light-sensitive materials, particularly light-sensitive materials for cameras; one is high sensitization exemplified by a film of ASA 400, and the other is an improvement in image quality so as to cope with miniaturization of films. In connection with the former high sensitization, various techniques have heretofore been studied, including a method of increasing the grain size of silver halide, a method of increasing the activity of couplers, and acceleration of development.
The sensitivity seems to have reached the uppermost limit as described in G. C. Farnell & J. B. Chanter, J. Photogr. Sci., Vol. 9, page 75 (1961) and thus no further increase in sensitivity can be expected even by increasing the grain size of the silver halide. Furthermore, various problems such as reduction of graininess are involved in increasing the grain size of silver halide. Extensive investigations have heretofore been made to increase the activity of couplers. This activation, however, cannot be achieved to an extent sufficient to contribute to the increase of sensitivity. On the contrary, it has a disadvantage of reducing the graininess. In connection with the acceleration of development, incorporation of various development accelerators such as hydrazines in an emulsion layer or a developer has heretofore been investigated mainly for black and white light-sensitive materials. However, addition of such development accelerators often results in an increase of fog and reduction of graininess, and is not suitable for practical use.
In view of the problems as described above, a coupler releasing imagewise a development accelerator or a foggant has been proposed. For example, U.S. Pat. Nos. 3,214,377, 3,253,924, and Japanese Patent Application (OPI) No. 17437/76 (the term "OPI" as used herein means a `published unexamined Japanese patent application") discloses a thiocyanic acid ion-releasing coupler which accelerates solution physical development. In addition, Japanese Patent Application (OPI) No. 150845/82 discloses a coupler releasing acylhydrazine, and Japanese Patent Application (OPI) No. 138636/82 discloses couplers releasing developing agents such as hydroquinone and aminophenols.
The above-described releasing groups, however, are inferior in the development acceleration or fogging action. Therefore, the development acceleration or fogging effect is difficult to obtain unless the couplers releasing such groups are used in large amounts. Furthermore, even if they are used in large amounts, the effect obtained is very small. Moreover, when the couplers are incorporated in a certain light-sensitive layer, they diffuse in another layer because of high diffusability of the releasing groups, causing the acceleration of development or fogging in the different layer. This is not preferred for color reproduction because of color mixing and further reduces the graininess.